Tuning control circuits



April 29, 1941.

Filed April 21, 1938 2 Sheets-Sheet 1 v INVENTOR- CAREL .1. VAN Loo/v ATTORNEY.

April c. J. VAN LOON\ 2,239,907

TUNING CONTROL CIRCUITS Filed April 21, 19:58 2 Sheets-Sheet 2 TOI. EAMPL.

INVENTOR.

CAREL .1 mm won BY AIITQRNEY.

Patented Apr. 29, 1941 TUNING CONTROL CIRCUITS Carel Jan van Loon, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application April 21, 1938, Serial No. 203,281 In Germany July 9, 1937 14 Claims.

This invention relates to a wireless receiving circuit with automatic volume control, and has for its object to remove the difliculty occurring in such receiving circuits that noises become manifest in the loudspeaker with great intensity when the receiving circuit is not tuned to a signal.

For this purpose it has already been suggested toprovide a receiving circuit with a so-called noise suppressor which in the absence of tuning suppresses sound reproduction. These known receiving circuits with noise suppression present, however, the disadvantage that it is very difficult to effect tuning since sound reproduction is had Withi'na narrow frequency spectrum only, and the listener has generally already. passed this frequency spectrum before he can stop the tuning knob. For obviating this disadvantage it was suggested to incorporate a magnetic brake, or the like, which upon'correct tuning being reached renders diflicult, or prevents, further rotation of the tuning means. This renders, however, the receiver'set substantially costlier.

According to the'invention, a sufficient noise suppression is obtained without tuning being rendered more difiicult by providing a mechanical, or electric, relay which is controlled by the received oscillations, and by which in the case of sufiiciently accurate tuning to the signal, the width of the frequency spectrum transmitted by the receiving circuit is altered discontinuously in such a manner that in the absence of tuning the highest frequencies of the audio-frequency spectrum to be reproduced are suppressed, and in the case of sufficiently accurate tuning all the frequencies that are comprised within this spectrum are transmitted.

The invention is based on the recognition of the fact that noises occurring between two signals during the tuning process generally have a very sharp sound nature so that these noises can be largely removed by suppression of the highest audio frequencies. This secures the important advantage over well-known noise suppressors that prior to correct tuning being obtained the listen- L ers attention is drawn by a distorted reproduction of the received signal to the presence thereof so that'he can stop the tuning knob in time. Correct tuning is quite distinctly indicated by a sudden variation of the sound tone so that tuning of the receiver according to the invention is most simple.

It has previously been suggested to provide receivers having automatic band width control which upon reception of feeble signals reproduce a narrow frequency band and upon reception of intense signals a wide frequency spectrum. With these receivers the sound tone is, however, altered continuously. Although these known circuits may result in some cases in weakening of the interfering noises, the distinct indication of the correct tuning which is characteristic of the circuit according to the invention is missing'due to the continuous variation of the sound tone. In addition, in the present circuit arrangement, ac cording to the invention, the reproduction quality prevailing in the absence of tuning has only to satisfy very low conditions so that in the absence of tuning it is possible to employ substantially more emcient suppression of the high audio frequencies than with the circuits having an automatic band width control, and thus to obtain substantially higher freedom from'interference. In the circuit arrangement according to the invention it is thus, for example, possible in the absence of tuning to suppress all the audio frequencies above 1000. cycles.

In the drawings:

Figs. 1 to 4 show respectively different embodiments of the invention.

Fig. 1 shows the parts of a superheterodyne receiving circuit which are necessary for an understanding of the invention. The signal oscillations in the antenna! are supplied to the highfrequency part 2 which comprises the local oscillator, the mixing stage, and in some cases a highfrequency amplifier. The intermediate frequency output voltage of the mixing stage is supplied to an intermediate frequency amplifier valve 5 by a band-pass filter constituted by the two circuits 3 and 4 which are inductively coupled. The output circuit of the valve 5 includes the first circuit of a second intermediate frequency band-pass filter which is constituted by the coupled circuits l5 and l, and supplies the diode contained in the valve 8 with the amplified intermediate frequency oscilla tions. occurs across the resistance 9 which is bridged by a condenser I ll that constitutes a short-circuit for the intermediate frequency. The low-frequency component of the voltage that occurs across the resistance 9 is supplied by a condenser I l, and by means of a tapping which is variable for the purpose of volume control, to the control grid of the amplifier system contained in the valve 8; the control grid is grounded by a leak resistance l2. The anode circuit of the valve 8 is coupled to the input circuit of the final valve [6 by means of a coupling element constituted by resistances l3 and I5 and a condenser M. The

The rectified output voltage of this diode anode circuit of the valve I6 includes the primary of an output transformer I7 whose secondary has a loudspeaker 2| connected to it. According to the invention the loudspeaker 2| has connected in parallel with it a condenser I8 which is connected in series with a tap 29 of a relay magnet I9. Upon the magnet I9 responding, the tap 29 is closed, and the condenser I8, which constitutes a short-circuit for the highest frequencies of the audio-frequency spectrum to be reproduced, is connected to the loudspeaker terminal so that the sound tone of the reproduced sound varies discontinuously.

The direct voltage component of the voltage occurring across the resistance 9 serves for automatic volume control, and is supplied for this purpose to the control grid of the intermediate frequency amplifier valve by a filter comprising the elements 22, 23 and 25, 25. In addition, the control voltage is also supplied to the valves included in the high-frequency network 2 by the lead 24. The electromagnet I9 is included in the anode circuit of the intermediate. frequency; amplifier valve 5, and is shunted by a condenser. 27 which constitutes a. short-circuit for the intermediate frequency.

In th absence of tuning of the receiver only a small direct voltage drop occurs across the resistance 9. In thiscase, the anode current of the valve 5 is high enough for the relay I9'to be attracted and for the highest audio frequencies to be suppressed by means of the condenser I8. As soon as, however, the receiver is tuned toa signal with suflicient accuracy the direct voltage drop across the. resistance 9 increases greatly, and the anode current. of the valve 5 is thus decreased to such an extent that the magnet I9 permits tap 20 to fall back and cut the condenser I 8 out: of circuit. Instead of the condenser I8 any other tone filter separating the highest audio frequencies may be used.

In the circuit arrangement shown in Fig. 1 the magnet I 9 is controlled by the control uoltage serving for automatic volume control. This kind of control has the disadvantage that, particularly in the case of intense signals, the relay may respond too early. In the circuit arrangement shown in Fig. 2 this disadvantage is obviated by the relay being controlled by the rectified output voltage of a particularly selective intermediate frequency channel.

In the circuit arrangement shown in Fig. 2 the intermediate frequency voltage occurring across the circuit 1 is supplied by a. coupling condenser 28 to the control grid of an additional amplifier valve 30 whose anode circuit includes a highly selective oscillatory circuit 3I tuned to the intermediate frequency. The voltage occurring across this oscillatory circuit is supplied by a condenser 32 to a diode contained in the valve 99. The direct'voltage across a leak resistance 39 is supplied by a filter 94, 35 and a resistance 29 to the control grid of the valve til. The relay I9 is included in the anode circuit of the valve 35). When the receiver is not tuned to a signal. a voltage does not occur across the circuit SI and the anode current of the valve 39 is consequently high enough for the relay tap 2G to be attracted. As soon as, however, the receiver accurately tuned to a signal a voltage occurs across the circuit 3H, and a substantial direct voltage drop is set up across the resistance 33 so that the anode current of the valve 38 is decreased to such extent that the relay tap 29 is released.

Due to the high selectivity of the circuit 3| the highest frequencies of the audio spectrum to be reproduced are not comprised in the low frequency voltage across the resistance 33. This fact can be made available for the purpose of the invention by connecting the low frequency amplifier of the receiver in the absence of tuning to the resistance 33, and by changing the connection over to the resistance 9 by means of the relay magnet I9 in the case of sufiiciently accurate tuning. This embodiment of the inventicn is shown in Fig. 3. In the circuit arrangement shown in Fig. 3 the relay magnet I9 is provided with contacts 31 and 38 to which the low-frequency voltages across the resistances 9 and 99 are supplied by condensers II and 36. The control grid of the valve 8 is connected to th tap 28. In the absence of tuning of the receiver the relay tap 29 is attracted, and the control grid of the valve 8 is connected to the re sistance 39. As soon as the receiver is tuned accurately to a signal, the relay tap 20 is released, and the control grid of the valve 8 is connected to the resistance 9.

Inthe circuit arrangement shown in Fig. 4 the sudden variation in sound tone upon correct tuning is obtained by a variation in band width of an intermediate frequency band-pass filter. The band-pass filter circuits 6 and I are coupled together so loosely in the absence of tuning that the band-pass filter has a high selectivity. The relay magnet I9 included in, the anode circuit of the valve 5 is so arranged that when the relay responds the. coils of the circuits 6 and I are displaced relatively to each other as is diagrammatically indicated by the arrow 39. In the absence of tuning the relay is energized, and the band-pass filter coils are coupled together very loosely. Upon a sufiiciently accurate tuning to a signal being obtained the relay armature I9 falls back, and alters the mutual position of the band-pass filter coils in such manner that the coupling increases and the selectivity of the band-pass filter is substantially decreased. Any device well known in the art can be used to restore armature I9 when magnet I9 is deenergized.

What is claimed is:

1. In a receiver of the type including a modulation signal carrier energy transmission network, a rectifier and a reproducer; the method which includes transmitting modulated carrier energy through said network, rectifying said transmitted carrier energy, reproducing the modulation frequencies of the rectified carrier energy, and greatly reducing the reproduction of the higher modulation frequencies in response solely to a departure of the said carrier frequency from a predetermined frequency value thereby to cause a distorted sound to appear at the reproducer to indicate detuning of the receiver.

2. In a receiver :of'thetype including a modula tion signal carrier energy transmission network, a rectifier and a reproducer; the method which includes transmitting modulated carrier energy through said network, rectifying said transmitted carrier energy, reproducing the modulation frequencies of the rectified carrier energy, and greatly reducing the reproduction of the higher modulation frequencies with the direct current voltage component of the rectified energy in response solely to a departure of the said carrier frequency from a predetermined frequency value thereby to cause a distorted sound to appear at the reproducer to indicate ,detuning of the receiver.

3. In a receiver of the type including a modulation signal carrier energy transmission network, arectifier and a reproducer; the method which includes transmitting modulated carrier energy through said network, rectifying said transmitted carrier energy, reproducing the modulation frequencies of the rectified carrier energy, and greatly reducing the reproduction of the higher modulation frequencies at said reproducer in response solely to a departure of the said carrier frequency from a predetermined frequency value thereby to cause adistorted sound to appear from the reproducer to indicate detuning of the receiver.

4. In a radio receiver, a high frequency amplifier tuned to a predetermined carrier frequency, a detector and an audio frequency transmission network, means responsive solely to a departure of the frequency of the amplified signals from said carrier frequency for increasing the amplifier output current, and means responsive to said increased current for greatly reducing the amplitude of the higher audio frequencies of the detected signals fed to said audio network thereby to cause the audio output of said audio network to be greatly distorted and to indicate said frequency departure.

5. In a radio receiver having a detector provided with an input circuit tuned to a desired carrier frequency, an audio network coupled to said detector for transmitting the audio frequencies of the detected signals, a rectifier having a highly selective input circuit, tuned to said carrier frequency, coupled to said first input circuit, and means responsive to said rectifier output electrically connected to said audio network to control transmission of said detected signals therethrough in a sense for substantially reducing the amplitude of the higher audio frequencies of said detected signals upon detuning of the receiver from said desired carrier frequency thereby providing an audible indication of said detum'ng.

6. In a radio receiver having a detector provided With an input circuit tuned to a desired carrier frequency, an audio network coupled to said detector for transmitting the audio frequencies of the detected signals, a rectifier having a highly selective input circuit, tuned to said carrier frequency, coupled to said first input circuit, means normally disconnected from the detector and said audio network for deriving a restricted band of audio frequencies from said rectifier, and means responsive to said rectifier output for substantially reducing the amplitude of the higher audio frequencies of said detected signals transmitted through said audio network by connecting said deriving means to the audio network instead of said detector.

7. In combination with an. audio-modulated carrier energy amplifier tuned to a desired carrier frequency, a detector for deriving from the amplified energy the audio modulation frequencies, and means to reproduce the said audio frequencies; the improvement which includes means of greater selectivity than said detector for deriving from the carrier energy a direct current voltage whose amplitude is dependent upon the frequency difference between the frequency of said carrier and said desired frequency, and additiona1 means responsive to the said direct current voltage and operable solely on the output of said detector for automatically regulating the intensityof the higher audio frequencies in the reproduc'ed frequencies. 1 3

8. In combination with an audio-modulated carrier energy amplifier tuned to a desired carrier frequency, a detector for deriving from the amplified energy for audio modulation frequencies, and means to reproduce the said audio frequencies; the improvement which includes means for deriving from the carrier energy a direct current voltage Whose amplitude is inversely dependent solely upon the frequency difference between the frequency ofsaid carrier and said desired frequency, and additional means responsive to the minimum amplitude of said direct current voltage for automatically reducing the intensity of the higher audio frequencies above 1000 cycles in the reproduced frequencies thereby to produce a highly distorted sound at the reproducing means to indicate audib-ly said frequency difference.

9. In combination with an audio-modulated carrier energy amplifier tuned to a desired carrier frequency, a detector for deriving from the amplified energy the audio modulation frequencies, and means to reproduce the said audio frequencies; the improvement which includes means for deriving from the carrier energy a direct current voltage whose amplitude is inversely dependent solely upon the frequency difference between the frequency of said carrier and said desired frequency, and additional means responsive to the minimum amplitude of said direct current voltage for automatically reducing the intensity of the higher audio frequencies in the reproduced frequencies thereby to produce a highly distorted sound at the reproducing means to indicate audibly said frequency difference, said additional means including a tone control device operatively associated with the detector output circuit.

10. A method of indicating the accuracy of tuning of a radio receiver which includes collecting audio modulated carrier signals, amplifying the collected signals, detecting the amplified signals to produce currents of audio frequency, and suppressing the higher audio frequency components of the detected currents above 1000 cycles upon solely a frequency departure of the frequency of the collected signals from a, desired carrier frequency thereby to produce a highly distorted sound output from the receiver.

11. In combination with an audio-modulated carrier energy amplifier tuned to a desired carrier frequency, a detector for deriving from the amplified energy the audio modulation frequencies, and means to reproduce the said audio frequencies; the improvement which includes means of greater selectivity than said detector for deriving from the carrier energy a direct current voltage whose amplitude is dependent upon the frequency difference between the frequency of said carrier and said desired frequency, and additional means responsive to the said direct current voltage and operable solely on the output of said detector for automatically regulating the intensity of the higher audio frequencies in the reproduced frequencies, said deriving means comprising a rectifier provided with a highly selective input connection to a point succeeding the carrier amplifier.

12. In combination with an audio-modulated carrier energy amplifier tuned to a desired carrier frequency, a detector for deriving from the amplified energy the audio modulation frequencies, and means to reproduce the said audio frequencies, the improvement which includes means of greater selectivity than said detector for deriving from the carrier energy a direct current voltage whose amplitude is dependent upon the frequency diiference between the frequency of said carrier and said desired frequency, and additional means responsive to the said direct current voltage and operable solely on the output of said detector for automatically regulating the intensity of the higher audio frequencies in the reproduced frequencies, said greater selectivity deriving means comprising an amplifier having an input connection to said detector input, a carrier-tuned rectifier coupled to the last named amplifier, said automatic regulating means being connected to the latter, and means for controlling the space current flow of said last amplifier in response to the output of said rectifier.

13. In a modulated carrier energy receiving system, a pair of rectifiers, each rectifier being provided with its own load impedance, said load impedances normally being electrically disconnected, a modulation voltage utilization network, means for selectively connecting either one of said load impedanceslt'o saidutilization network, said rectifiers having carrier input circuits of different selectivity, and means responsive to rectification by one of said rectifiers for causing said connecting means to connect the load impedance of the other rectifier to said utilization network.

14. In a modulated carrier energy receiving system, a pair of rectifiers, each rectifier being provided with its own load impedance, said load impedances normally being electrically discon nected, a modulation voltage utilization network, means for selectively connecting either one of said load impedances to said utilization network, said rectifiers having carrier input circuits of different selectivity, means responsive to rectification by one of said rectifiers for causing said connecting means to connect the load impedance of the other rectifier to said utilization network, and said one rectifier having a carrier input circuit of substantially higher selectivity than the input circuit of the other rectifier.

CAREL JAN VAN LOON. 

